An analysis of the shapes of ultraviolet extinction curves. IV. Extinction without standards

In this paper we present a new method for deriving UV through IR extinction curves, based on the use of stellar atmosphere models to provide estimates of the intrinsic (i.e., unreddened) stellar spectral energy distributions ( SEDs), rather than unreddened ( or lightly reddened) standard stars. We show that this ''extinction without standards'' technique greatly increases the accuracy of the derived extinction curves and allows realistic estimations of the uncertainties. An additional benefit of the technique is that it simultaneously determines the fundamental properties of the reddened stars themselves, making the procedure valuable for both stellar and interstellar studies. Given the physical limitations of the models we currently employ, the technique is limited to main-sequence and mildly evolved B stars. However, in principle, it can be adapted to any class of star for which accurate model SEDs are available and for which the signatures of interstellar reddening can be distinguished from those of the stellar parameters. We demonstrate how the extinction without standards curves make it possible to ( 1) study the uniformity of curves in localized spatial regions with unprecedented precision, ( 2) determine the relationships between different aspects of curve morphology, ( 3) produce high-quality extinction curves from low color excess sight lines, and ( 4) derive reliable extinction curves for mid to late B stars, thereby increasing spatial coverage and allowing the study of extinction in open clusters and associations dominated by such stars. The application of this technique to the available database of UV through IR SEDs, and to future observations, will provide valuable constraints on the nature of interstellar grains and on the processes that modify them, and it will enhance our ability to remove the multiwave-length effects of extinction from astronomical energy distributions.